Hot wire probes for measuring the rate of fluid flow have been known at least since the early 1900's. The hot film probes which were developed in the mid-1950's were an improvement upon the earlier hot wire devices. Those hot film probes had better dynamic response, superior mechanical properties, and higher signal-to-noise ratios than the hot wire probes. For a discussion of the early hot film probe, see the monograph titled "The Hot-Film Anemometer: A New Device For Fuuid Mechanics Research" by S. C. Ling and P. G. Hubbard in the Journal Of The Aeronautical Sciences, September 1956, pp 890 and 891.
The hot film probe was improved in subsequent years and some of those improved devices are described in U.S. Pat. Nos. 3,352,154; 3,900,819; and 4,024,761. From those patents and from other technical literature dealing with hot film probes, it is evident that efforts were made to improve the response of the probe to changes in the fluid's rate of flow by improving the sensitivity of the probe--that is, by improving the ability of the probe to respond to small changes in the flow rate. Despite the improvement efforts, the response time of the hot film probe to changes in flow rate, without degrading the accuracy of measurement, has remained relatively slow. In recent times, a need has arisen for hot film probes that accurately respond to flow rate changes with a rapidity unavailable from presently known hot film probes.
U.S. Pat. No. 4,024,761, in FIGS. 7 and 8, shows a thin film probe having three parallel heated elements arranged side by side and held together by thermally insulative bridges closing the gaps between adjacent elements. That three element probe is an improvement upon the dual element probe shown in FIGS. 12 and 13 of U.S. Pat. No. 3,900,819 because the three element probe provided increased sensitivity compared to the dual element sensor. That increase in sensitivity was attributed to the heat provided by the middle element to the leeward sensor element. It is recognized in U.S. Pat. No. 4,024,761 that the spacial response of the dual element sensor can be selectively altered by disposing a third heated element between the two sensors. Specifically, the patent states that the third element can be used to thermally bias the pair of sensor elements to improve their cosine law agreement when used as direction sensing devices.